Designs for turbocharged two stroke cycle internal combustion engines have been known for a long time. The turbocharger is used to draw in outside air and pressurize it to a desired pressure. The pressurized air is then supplied to the intake ports of the cylinders in the engine. When the piston in a cylinder is near the bottom of its stroke, the intake ports are open which allow the pressurized air to enter the cylinder and push exhaust gas out of the cylinder. Fresh air is left behind in the cylinder for the next combustion cycle. After the piston begins its upward movement, a fuel injector injects fuel into the top of the cylinder.
A drawback of prior two stroke cycle engine designs becomes apparent when one tries to start them. Since the turbocharger uses the exhaust gas from the engine for its source of power and since there is no appreciable exhaust gas during startup, the turbocharger lacks the exhaust gas needed to operate efficiently and so there is no pressurized air available at the intake ports of the cylinders during startup.
Common solutions to this startup problem have included "Roots blowers" and other mechanical superchargers and scavenge pumps which are mechanically coupled to the crankshaft of the engine. The rotational movement of the crankshaft by the starter motor drives the Roots blower, supercharger or pump which then provides the startup air pressure to get the two stroke cycle engine started. Once started, the turbocharger receives the exhaust gas needed to become operational and the turbocharger thereafter supplies most of the compressed air for normal operation of the engine.
As noted, however, such a design requires a fairly complicated blower, supercharger or pump which has a substantial cost. Further, when the engine is for use in boats or airplanes, the additional weight of the blower, supercharger or pump is particularly undesirable.
Another drawback of conventional designs for two stroke cycle engines is that they include separate cylinder heads, separate cylinder walls, and/or a separate crankcase. Each of the separate components must be joined together with suitable fasteners, such as bolts, which add significant weight to the engine design. At each joint between components, a gasket is often used to prevent leaks. The gasket is sandwiched between opposing faces which are drawn tightly together. To insure a good seal, the opposing faces are typically thicker than adjoining materials. The increased thickness of the opposing faces adds further weight to the design.
Accordingly, there is a need for a lightweight, high output two stroke cycle turbocharged engine which starts reliably without use of heavy mechanical blowers.